1. Field of the Invention
This invention relates to an electron microscope wherein the density of electron beam projected on a sample of material being examined can be detected and also controlled.
2. Description of the Prior Art
In an electron microscope, a beam of electrons is projected onto a sample to be examined and a magnified image of the sample is produced by use of the electrons scattered by or transmitted through the sample, so that the irradiation of the sample by the electron beam is essential for examination.
However, the irradiation of the sample by the electron beam causes two inherent problems: contamination in the sample and damage of the sample. Many researches have been reported on the contamination in samples and successful measures have been found to prevent such contaminations. For example, one of the measures is to cool the sample chamber.
On the other hand, there have been few countermeasures against the damage of the sample. The sample damage can be classified roughly into two sorts: the damage due to the irradiation of the sample by an electron beam of excessively high density and that derived from an excessive amount of electric charges accumulated due to long time irradiation. Both the damages tend to change the inherent property of the sample and therefore in order to make a precise examination of the sample, it is necessary to measure and control the density of the electron beam applied to the sample during examination.
The measurements of the electron beam density are often done in the field of electron microscopes when microscopic images are to be photographed. That is, the density of the electron beam has a certain relation to the brightness in photographing and the beam density as one of conditions in photographing is measured. As a result of developing such a measurement, there has been proposed a method wherein in order to enable photographing under a constant brightness independent of the magnifying power of electron microscope the measured density of the electron beam is compared with a signal representing a desired brightness, and the lens system of the electron microscope is controlled in accordance with the result of the comparison. Such a method is disclosed in, for example, U.S. patent application Ser. No. 712,664 (Mar. 25, 1977).
The above described method is indeed intended to automatically control brightness of the images in photographing, but not to be used as a measure for preventing damage of the sample.